Sounds like your new ram is not compatible with your present set-up...
It is most likely returnable or exchange for compatible ram (same ram)
I have a white box Pentium 4 CPU with XP professional service pack 2 build 2600 installed. The motherboard is an intel D865GLC with 512 MB RAM installed in Dual channel mode with 1 256 MB stick of ram in slot zero of bank 1 and 1 256 MB stick of ram in slot zero of bank 2. This memory is dual channel compatible. I tried to add another 2 sticks of the same type ram but from a different manufacturer.
Here's the problem: I can use any 2 of the 4 sticks of ram together, both from the same manufacturer or 1 from 1 manufacturer and 1 from the other for a total of 512 MB. Everything works fine. I can place one stick in slot 0 of bank 1 and the other in slot 0 of bank 2 or I can place 1 in slot 1 bank 1 and 1 in slot 1 of bank 2 with no problems. I can even place a 3rd stick of the same type ram in any open slot and all's fine (except the machine complains that this is not an optimal memory setup). Memtest86 reports no errors with any of the above configurations. But if I add in the 4th stick to operate in dual channel mode with a total of 1024 MB memory the machine locks up or reboots at random and Memtest86 reports errors from the lower memory, such as 391 MB all the way up through the 900's and the 1000s. I have tried reseating all the memory but to no avail. It hates having 4 sticks of ram no matter what. HELP!